Thinking about things that most people need to be warned about. Don't have to be car related but mine are:
1. snow scraper brushes --- holy crap!! these cheap things for sale in the supermarket and auto parts stores with a ice scraper and those harsh brushes should be outlawed. It's amazing that even dumb people wouldn't touch their cars with these the rest of the year with the car dry, but some really smart people take these things to their car with an inch or 2 of snow on them!! Scratches the hell out of them. I have told people while they were using them and they never really thought about the paint under the snow. It's not an issue for you guys down south but anywhere it snows these things are doing damage!!
2. polished aluminum wheels ---- car guys know that these things take much effort to maintain. Unfortunately nobody at the tire/wheel store bothers to tell the average guy/gal. They see these awesome shiny rims, cheaper then the chrome ones and buy them. I see these people all over, Cars in pretty good shape but their wheels old, chalky and no shine because they just drive the car and wash it once and a while. I'm sure had they know BEFORE buying that that would happen they would have made a different choice. Most are so far gone I wouldn't even try to polish them back!!!
I'm sure I'll think of more later............